Nicotinic Acetylcholine Receptors in Muscle

Abstract

Acetylcholine is the neurotransmitter carrying the excitatory signal at the neuromuscular endplate from the presynaptic to
the postsynaptic side across the synaptic cleft. Together with acetylcholinesterase, the nicotinic acetylecholine receptor
regulates the intensity and duration of the signal at the muscle.

The nicotinic acetylcholine receptor. (a) Electron‐microscopic picture of the membrane bound receptor (left: longitudinal
section; right: cross‐section showing the pentameric structure. (b) Schematic of the receptor protein showing the pentameric
complex in its membrane environment (left) and a summary of the known functional domains (right): the helices depict the wall
of the ion channel proper; the ellipses represent the location of the ligand‐binding sites at the interface between subunits.
(c) Schematic representation of the transmembrane folding of the receptor subunits: M1–M4 represent the four membrane‐spanning
sequences connected by extramembrane loops. N and C are the N‐ and C‐termini, respectively, of the polypeptide chain.

Figure 4.

Schematic representation of AChR aggregation in the postsynaptic membrane triggered by agrin. Synaptic differentiation involves
at least five steps: (1) redistribution of AChRs, which are initially distributed on the entire myotube surface, to the subsynaptic
part of the postsynaptic membrane; (2) increased transcription by synaptic nuclei of mRNAs encoding AChR subunits; (3) decreased
transcription by extrasynaptic nuclei; (4) rearrangement of membrane cytoskeleton; (5) retrograde signal from the postsynaptic
to the presynaptic side. ‘AAAA’ indicates polyadenylation. Reproduced from Hoch, , with permission from the author.